This invention relates to the centrifuge field, and particularly to the sample retaining means used in the bores, or cavities, of a centrifuge rotor.
The advantages of the present invention are primarily intended for use in centrifuge rotors having bores which are inclined with respect to the vertical rotational axis, so that the centrifugal force has a component urging the sample toward the bottom of the bore. However, the invention may also be useful in conjunction with centrifuge rotors having vertical sample-containing bores.
Furthermore, the present invention appears to have its primary advantages in conjunction with the use of "Quick Seal" sample-containing tubes, which are tubes having their cover areas formed integrally with their bodies, and sealed by fusion of a nipple, or neck, after it has been used for insertion of the fluid sample. Such tubes have proved to be highly advantageous, as compared with earlier open top tubes, which had to be sealed with separate caps and which therefore had serious sealing problems.
The invention of "Quick Seal" tubes is disclosed in Nielsen Application Ser. No. 912,698, titled "An Integral One Piece Centrifuge Tube", filed on June 5, 1978, and assigned to the assignee of the present application.
Since Quick Seal tubes are thin-walled vessels in which the cover portion is integral with the body portion, the forces developed by centrifuge operation have a tendency to deform the upper portion of the tube. Such forces are due both to the hydraulic pressures inside the tube which act on the tube during centrifugation, and to the "buckling" effect on the inner, or centripetal, portion of the tube if significant amounts of air are enclosed in the tube, either entrained in the liquid material or left in the tube because the liquid does not fill it.
In order to prevent deformation of Quick Seal tubes, certain precautions must be taken, particularly in providing support for the upper surface of the tube. In bores which are obliquely oriented with respect to the rotor axis, an ideal arrangement comprises a floating supporting cap engaging the top of the tube, even though such a cap is not required for closing, or sealing, the tube. The particular advantage of a floating cap is that centrifugal force will thrust it against the tube top, so that its pressure on the tube top will tend to prevent deformation of the tube. Because such a cap is floating it will firmly engage the top of the tube regardless of the distance between the tube top and the upper end of the rotor cavity, or bore, which contains the tube.
As shown in FIG. 12 of Nielsen Application Ser. No. 912,698, a floating cap, or plug, may lie on top of the tube in the obliquely-oriented cavity, relying on centrifugal force to maintain the cap, or plug, in engagement with the tube, and thus to provide support for the tube during centrifugation. A very significant advantage of floating caps used in inclined bores is that the counterbore can be completely eliminated. This will make possible rotors with higher performance and/or greater tube volume because of improved stress conditions near the upper portion of the rotor.
As the full significance of the tube-supporting function of the floating cap has been more fully recognized, and as performance tests have been made on the combination of Quick-Seal tubes with such floating caps, structural problems have been encountered and analyzed relating to the shape of the floating cap shown in FIG. 12 of the Nielsen application. Specifically, it has become apparent that the floating cap must be so designed as to be structurally self-sufficient in avoiding permanent deformation by the powerful centrifugal and hydraulic forces generated in the centrifuge.
The tube supporting function of the cap has involved extreme stressed on the floating caps, which have caused them to deform into substantially oval or elliptical shapes, and to take a set in the deformed shapes, rendering the caps difficult to extract from the rotor cavities, and useless for subsequent centrifuge operations.
Another aspect of floating caps which has been the subject of significant improvement since disclosure of the concept in the Nielsen Application is the selection of material for such caps, particularly with reference to its density. In the event of failure of a tube, it is desirable that the position shift of the cap be minimized in order to avoid rotor imbalance during centrifugation. Also the cap material should not be such as to score the walls of the tube cavity.
In general, the present invention is intended to solve the structural deformation and related problems encountered in using the combinations of Quick Seal tubes with floating caps, which are particularly useful in conjunction with centrifuge rotors having obliquely-oriented sample-containing bores.